CHĀNDOGYA UPANISHAD · 3.12.5
सैषा चतुष्पदा षड्विधा गायत्री ।
तद् एतद् ऋचाभ्यनूक्तम् ॥
saiṣā catuṣpadā ṣaḍvidhā gāyatrī ।
tad etad ṛcābhyanūktam ॥
That Gâyatrî has four feet and is sixfold. And this is also declared by a Rik verse (Rig-Veda X, 90, 3):--
F. Max Müller · The Upanishads, Sacred Books of the East 1 (1879)